Film attenuator

ABSTRACT

An electrical film attenuator or other film resistor network comprising an insulating substrate having adherently mounted thereon a single film resistive element, said element being provided with at least three terminals in electrical contact therewith and, on said element and in electrical contact therewith, an electrically conductive layer dividing up said element into three distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of said terminals, one of which being an input and the other an output terminal, the third portion of said electrical resistive element being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said first two portions being in electrical contact with only one of said two terminals, the third portion being electrically connected to the remaining terminal or terminals.

I United States Patent 3,680,0 13 Pye 1 July 25, 1972 [54] FILMATTENUATOR 3,594,679 7/1971 Seay ..338/l 95 [72] inventor: Walter ArnoldPye, Morpeth, Northumberlandt England 3,521,201 7/1970 Veteran ..333/81R [73] Assignee: Welwyn Electric Limited, Northumberland, England tPrimary Examiner-Lems H. Myers Assistant Examiner-A. T. Grimley Filed: 11971 Allorne vMcDougall, Hersh & Scott [2i] Appl. No.. 115,244 ABSTRACTAn electrical film attenuator or other film resistor network [30]Foreign Application Prhrny Data comprising an insulating substratehaving adherently mounted Feb. 27, I970 Great Britain ..9,720/70 thereona single film resistive element, said element being provided with atleast three terminals in electrical contact 52 U.S. Cl ..333/81 R,29/620, 323/94 H, therewith and, on said element and in electricalContact 33 195 333 309 333 314 333/325 therewith, an electricallyconductive layer dividing up said 51 Int. Cl. ..H01 1/22 oiomottt intothree distinct portions two oiwhioh will have the 58 Field of Search.....338/l95, 307, 308, 309, 31 1, Same electrical resistance value afterremoval of pan of Said 338/314, 325; 323/94 R, 94 H; 333/81 R 31 C 81element at a locus midway between two ofsaid terminals, one 29/620 ofwhich being an input and the other an output terminal, the third portionof said electrical resistive element being shaped [56] Reierences Citedso as to enable its electrical resistance value to be adjusted byremoving part of it, each of said first two portions being in UNITEDSTATES E T electrical contact with only one of said two terminals, thethird portion being electrically connected to the remaining terminal3,599,l25 8/197] Yoshlda ..333/81 R or termina1s 3,329,921 7/1967 Badelt..338/325 X 3,573,703 4/1971 Burks ..338/l95 X 8 Claims, 18 DrawingFigures PATENTED 3,680,013

SHEET 1 OF 4 A EJGJ CL V INPUT R3 OUTPUT INPUT R15 OUTPUT RB R14 SHEET 2[IF 4 FIGZf.

PATENTEDJULZS I972 FIGZCI.

PATENTEU LZ I973 3,680,013

sum 4 BF 4 FILM ATTENUATOR This invention relates to electrical filmresistor networks, especially electrical attenuators and a process fortheir production. Attenuators are combinations or networks of fixedelectrical resistors arranged and interconnected in such a manner that,when inserted into the input line of an electrical device or circuit,they reduce the input power to a desired level while maintaining thenormal characteristic impedance of the device or circuit.

Attenuators, on being incorporated in an electrical circuit, providewhat is known as insertion loss" which is a relationship of electricalpower and is measured in decibels or nepers (l neper 8.686 decibels);they may be used, for example, in communication systems and in systemswhere it is necessary to maintain a constant level of power when theload is altered.

Two basic kinds of attenuators are known and these are termed balancedand unbalanced attenuators respectively. An unbalanced attenuator isgenerally constituted by three electrical resistors arranged andinterconnected in either Tee configuration or Pi configuration.

FIG. la of the accompanying drawings illustrates a known example of anunbalanced T-configuration, comprising resistors R,, R and R where R, R

FIG. lb of the drawings illustrates a known example of an unbalanced Pi"configuration, comprising resistors R R, and R where R R Balancedattenuators are generally constructed from four or five resistors; theymay be arranged in a square (Balanced Pi") configuration (an example ofwhich is illustrated in FIG. 1c of the drawings, consisting of resistorsR R R R, where R, R and R R,,,) or in an H (Balanced T") configuration(an example of which is illustrated in FIG. 1d, consisting of resistorsR,,, R R,,,, R,., and R all of which have the same resistance value).

In the manufacture of an electrical attenuator it is necessary to ensurethat both the insertion loss and the input and output impedancescorrespond to the required nominal values for these parameters withinstated tolerance limits.

When utilizing discrete resistors in the construction of attenuators,the resistors are manufactured with the desired values and tolerancesand then interconnected in the required configuration, input and outputterminations being provided. By means of this technique, the resultingnetwork can be made to provide the correct insertion loss relative tothe desired characteristic impedance with the necessary and stipulateddegree of accuracy.

Attenuators may also be manufactured by so-called flat film techniques,whereby thick or thin film resistor elements and their associatedinterconnecting conductors are deposited on flat substrates be means oftechniques such as electroless plating, vacuum evaporation, or screenprinting of cermet materials which are subsequently fired at a hightemperature. Attenuators of all the configurations described above maybe produced in this way. Attenuators produced by these flat filmtechniques suffer from the great disadvantage that the adjustment of theresistance values of individual film resistors which is required has tobe carried out on the network of deposited resistor films and it isdifficult to adjust the value of a single resistor since measurementsare normally made across the input or output terminations of theattenuator and the presence of more than one resistor causes confusion.Thus it is normal practice to break the attenuator circuit at someconvenient point, adjust each resistor to the required value and finallyrepair the break in the circuit. Adjustment of each resistor is carriedout by removing a portion thereof using, for example, a jet of aircarrying particles of an abrasive material, a laser beam or any suitablegrinding means. This procedure is slow since it is usually necessary toadjust at least three resistors. A further disadvantage exists, in thatif one of the first resistors in an attenuator is incorrectly adjusted,the other resistors may have to be adjusted to other than the preferredoptimum value for which the deposited film was originally designed.

It is a purpose of the present invention to minimize or overcome one orboth the disadvantages of the previously known methods of making flatfilm attenuators.

For clarification of the theoretical background to the inven tionreference is made to FIGS.2a, 2b, 2c, 2d, 2e and 2f of the drawings.

If one considers a film D of resistive material in the form of asymmetrical Tee" having uniform electrical sheet resistance and providedwith contacts A, B and C at each extremity as illustrated in FIG. 2a,then on connecting a pair of input lines to contacts A and C and a pairof output lines to contacts B and C, an unbalanced attenuator with equalinput and output impedances has been constructed. The equivalentcircuit, therefore, of such an attenuator is that shown in FIG. la andthe resistance measured between contacts A and B will be represented bythe sum of resistors R R,. In the embodiment represented by FIG. 2a, theregion of the resistance film situated between contacts A and B andrepresented by a central line of current flow B will in large part beequivalent to the two resistors R, R but it should be noted that allother regions of the resistive film will contribute to the measuredresistance between A and B. Similarly the resistance measured betweencontacts A and C as represented again by central lines of current flow(F F) will correspond to the sum of resistors R, R;,, and the resistancemeasured between contacts B and C, as represented by lines of currentflow (G' G), will correspond to the sum of resistors R R;,. The shuntresistor R is represented to a large measure by the resistive materialin the region of the central paths of current flow F and G withappropriate contributions from F and G. As paths E, (F F), and (G G) areidealized representations providing general indications of the directionof currents flowing in the overall resistive film, there is no absolutedistinction or perfect separation between them and in particular it isnot possible to equate areas of the resistive film to the equivalentresistors R,, R and R The electrical center of the equivalent circuit(i.e., the junction between R,, R and R in FIG. la) is not identified byany single point on the resistive film. Thus it is not possible to carryout an adjustment to the resistance value of those areas of the filmwhich represent the sum of (R, R i.e., as represented mainly by the filmin the region of path E, without at the same time affecting theresistance values of those areas of the film representing R For example,if one attempted to increase the resistance between contacts A and B byremoving a strip of film from the edge of D between A and B as shown inFIG. 212, then this would have some measurable effect on the measuredvalue of resistance between A and C and between B and C. Similarly theshunt resistance of the attenuator consisting largely of the currentpaths (F F) and (G G) cannot be increased in value by narrowing the limbof the resistive film in contact with C as shown in FIG. 20 withoutaffecting the resistance measured between A and B.

If a region M of conducting material is deposited on film D as shown inFIG. 2d so that it covers the full width of that limb of D whichterminates at C, then as a result a main line of current flow,represented by .I, can exist between C and M and two main lines ofcurrent flow, represented by L and (H K), can lie between A and B, andthe attenuator of the present invention is produced. The isolatedconductor region M is symmetrically situated with respect to contacts Aand B and is substantially equivalent to the junction between resistorsR,, R and R in FIG. la. M therefore defines the electrical center of theportion of resistive film lying between A and B (cf. the electricalcenter of resistors R, and R in FIG. la). Adjustment of the seriesresistance between A and B, which P comprises, may be carried outwithout affecting the value of the shunt resistance which N comprisesand vice versa. The strip N of resistive material is convenientlydeposited in such a pattern as to enable adjustment of the resistancevalue between C and M to be readily carried out. A top hat"configuration is shown for N in FIG. 2e, an increase in the, resistancevalue between C and M being achieved by removing part of the film in thedirection shown as Q. This adjustment, shown in FIG. 2f, may

be effected without affecting the resistance value of the series path P.Referring to FIG. 1a, the attenuator of the invention has means foradjusting R without affecting R, and R If part of the resistance film Pexactly midway between A and B is removed, or if a strip of film ofconstant width is removed from the whole length of P between A and B, asshown in FIG. 2f, then this will result in an increase in the totalresistance of the series path P without altering the electrical balanceof the two series resistors which P comprises and without affecting thevalue of the shunt resistance portion N. Thus, referring to FIG. la, theattenuator of the invention enables the series leg R, and R to beadjusted in resistance value while maintaining the balance between R,and R, and without affecting R It is seen that, in this way, only twosimple adjustments are required for an unbalanced attenuator and oneadjustment is not affected by the other. Two such attenuators, connectedtogether by their common terminals, form a balanced attenuator. Thebalanced attenuator so produced has twice the characteristic impedanceof the unbalanced halves. Only three adjustments are required for abalanced attenuator of the present invention since it is possible tocombine the resistance path N of each unbalanced half" to form a singleresistance path lying between two electrical center conductors M.

The invention is thus based on the finding that it is possible to make afilm attenuator by dividing up a single film resistive element with anelectrically conductive layer in electrical contact with said resistiveelement in a suitable position on the element; the conductive layer mustbe placed in such a way and the resistive element itself must be of ashape such that three distinct resistive element portions result, two ofwhich have the same resistance value, said portions being capable ofhaving their resistance value adjusted by removing part thereof. It willbe appreciated that in some cases, i.e. when it is desired to produce abalanced attenuator, two additional distinct portions of the resistiveelement must be provided and we have found that this can be done bymeans of a further electrically conductive layer applied to the singlefilm resistive element.

As will become apparent hereinafter, the above mentioned two portions ofthe single film resistive element which portions have the sameresistance value, could be said to be a single resistor, but since theyfulfill the function of the two resistors (in FIG. la these areindicated as R, and R of equal value of the classical film attenuator,elsewhere herein they are considered to be two portions which have thesame resistance value, especially as the region of the single filmresistive element concerned fulfills the function of two resistors ofequal resistance value connected in series.

The present invention thus provides an electrical film attenuator orother film resistor network comprising an insulating substrate havingadherently mounted thereon a single film resistive element, said elementbeing provided with at least three terminals in electrical contacttherewith and, on said element and in electrical contact therewith, anelectrically conductive layer dividing up said element into threedistinct portions two of which will have the same electrical resistancevalue after removal of part of said element at a locus midway betweentwo of said terminals, one of which being an input and the other anoutput terminal, the third portion of said electrical resistive elementbeing shaped so as to enable its electrical resistance value to beadjusted by removing part of it, each of said first two portions beingin electrical contact with only one of said two terminals, the thirdportion being electricallyconnected to the remaining terminal orterminals. The present invention further provides an electrical filmattenuator or other film resistor network comprising an insultingsubstrate, adherently mounted thereon a single film resistive element,said element being provided with four terminals in electrical contacttherewith and, on said element and in electrical contact therewith, twoelectrically conductive layers in such a way as to divide up the elementinto five distinct portions two of which will have the same electricalresistance value after removal of part of said element at a locus midwaybetween a pair of the terminals and two further of which will have thesame electrical resistance value after removal of part of said elementat a locus midway between the remaining pair of the terminals, saidfirst pair and second pair of terminals each being an output and aninput terminal, the fifth portion extending from one to the other ofsaid two electrically conductive layers and being shaped so as to enableits electrical resistance value to be adjusted by removing part of it,each of said four terminals being associated with only one portion ofsaid two sets of two portions having the same electrical resistancevalue.

The present invention also provides a method of producing an electricalfilm attenuator or other film resistor network, which comprisesproviding an insulating substrate, adherently mounting thereon a singlefilm resistive element, providing said element with at least threeterminals in electrical contact therewith and providing on said elementand in electrical contact therewith an electrically conductive layer insuch a way as to divide up the element into three distinct portions twoof which will have the same electrical resistance value after removal ofpart of said element at a locus midway between two of the terminals, oneof which being an input and the other an output terminal, the thirdportion being shaped so as to enable its electrical resistance value tobe adjusted by removing part of it, each of said first two portionsbeing in electrical contact with only one of said two terminals, thethird portion being electrically connected to the remaining terminal orterminals.

The above method is suitable for the production'of both balancedandunbalanced attenuators. In the case of an unbalanced attenuator it issufficient to provide three distinct portions of the resistive elementand a total of three terminals, two of which will serve as separateinput terminal and output terminal respectively, the third terminalserving as bothinput and output terminal. In the caseof a balancedattenuator, however, it is necessary to divide up the resistive elementso as to provide two further distinct portions (i.e., making fivedistinct portions in all) and this is done by applying to the singlefilm resistive element a further electrically conductive layer.

The present invention thus also provides a method of producing anelectrical film attenuator or other film resistor network, whichcomprises providing an insulating substrate, adherently mounting thereona single film resistive element, providing said element with fourterminals in electrical contact therewith and providing on said elementand in electrical contact therewith two electrically conductive layersin such a way as to divide up the element into five distinct portionstwo of which will have the same electrical resistance value afterremoval of part of said element at a locus midway between two of theterminals and two further of which will have the same electricalresistance value after removal of part of said element at a locus midwaybetween the remaining of the terminals, said first two and second twoterminals being one out put and one input terminal, the fifth portionextending from one to the other of said two electrically conductivelayers and being shaped so as to enable its electrical resistance valueto be adjusted by removing part of it, each of said four terminals beingassociated with only one of said two sets of two portions having thesame electrical resistance value.

From the above it is seen that the film attenuator of the inventioncomprises an insulating substrate having adherently mounted thereon asingle film resistive element provided with at least three electricalterminals and at least one electrically conductive film area inelectrical contact therewith in such a way as to divide up the resistiveelement into at least three distinctive portions. In the case of anunbalanced film attenuator, as will be seen from the above, there areonly three electrical terminals and three distinct portions of thesingle film resistive element having thereon one electrically conductivelayer. However, in the case of a balanced attenuator, there are fourterminals, five distinct portions of the single film resistive elementand two electrically conductive layers.

From another point of view, the film attenuator of the invention may beconsidered to have the allow following features:

a. A single film resistive element attached to a conductor patternconsisting of several distinct electrically conductive layers (includingat least one layer within the area of the resistive element) on aninsulating substrate; it is possible to apply the resistive elementfirst and then the electrically conductive layers or vice versa.

b. The resistive element must be adapted to enable it to be made into abalanced or unbalanced attenuator having the desired insertion loss andcharacteristic impedance.

c. Termination layers, the number of which depends on whether theattenuator is balanced or unbalanced.

The precise geometrical arrangement of the pattern of all the portionsof the resistive element and the conductive layers is dependent upon theinsertion loss required and upon the total area available and theoverriding requirement relative to power dissipation in the resistiveelement; for the sake of convenience, in general, the various portionsof the resistive element will have the shape of geometrical figureshaving a line of symmetry, but in all cases said resistive elementconsists of:

i. For an unbalanced attenuator: three termination conductive layersconstituting the input and output contacts of the attenuator, one ofthese layers being common to the input and output, and one furtherconductive layer in electrical contact with and situated within theresistive element in such a way that a line of symmetry can be drawnthrough it which is equidistant from the two termination layers neitherof which is the common termination layer.

ii. For a balanced attenuator: four termination layers constituting twoinput and two output contacts of the attenuator, the further conductivelayer mentioned at (i) and another, similar electrically conductivelayer (also situated within the resistive element) through which a lineof symmetry can be drawn in manner similar as mentioned at (i).

The single film resistive element, which is in contact with the three orfour termination layers and the one or two said further conductivelayers, provides:

1. One series resistance path between two termination layers, neither ofwhich is the common termination layer, in the case of an unbalancedattenuator, or two series resistance paths, one between eachcorresponding input and output termination layer, in the case of abalanced attenuator.

2. A resistance path between the electrical center of the seriesresistance path and the common termination layer, in the case of anunbalanced attenuator, or a resistance path between the electricalcenter of each of the two series resistance paths in the case of abalanced attenuator, the said further conductor layer or layersdefinings the said electrical center or centers of the series resistancepath or paths.

3. A first locus at which the resistance value or values of the seriesresistance path or paths may be adjusted without affecting theelectrical balance of the series resistance path or paths and a secondlocus at which either the resistance value of the resistance pathbetween the electrical center of the series resistance path and thecommon termination layer, or the resistance value of the resistance pathbetween the electrical center of each of the two series resistancepaths, may be adjusted without affecting the adjusted value of theseries resistance path or paths.

When the film attenuator is processed to give it the required nominalvalues for the insertion loss and input and output impedances, anadjustment of the resistive film element is made; the first step of thisadjustment comprises removing part of the resistive film midway betweenan input terminal layer and its corresponding output terminal layer orremoving a strip of resistive film of uniform widthfrom the entire edgeof the film between each input termination layer and its correspondingoutput termination layer, i.e., one operation for an unbalancedattenuator (because one of the three termination layers constitutes acommon input/output terminal) and two operations for a balancedattenuator.

The second step in the process of adjustment consists of removing partof the resistive film to etfectively reduce the width and increase thelength of the resistance path lying either between the electrical centerof the series resistance path and the common termination layer, for anunbalanced attenuator, or between the electrical centers of the twoseries resistance paths, for a balanced attenuator. The importantfeature of this second adjustment step is that, in addition to makingnormal resistance measurement during the adjustment process, measurementof the actual attenuation may be made so that compensation for minorerrors during the first adjustment step may be provided.

The attenuators of the invention have equal input and output impedance.

The attenuator of the invention requires adjustment of the singleresistive film in only two regions, in unbalanced form, to provide therequired electrical characteristics, compared with the adjustment ofthree separate elements required with unbalanced film attenuators of theprior art. In balanced form, the attenuator of the invention requiresadjustment of the single resistive film in only three regions, comparedwith the adjustment of four or five separate elements required withbalanced attenuators of the prior art. In the present invention eachsubsequent adjustment operation has no efiect of those previouslycarried out.

The geometry of the conductor pattern determines the ultimate insertionloss of the attenuator, while the electrical resistivity and shape ofthe resistor pattern determines the characteristic impedance. Tomanufacture an attenuator of a particular insertion loss andcharacteristic impedance, the conductor pattern appropriate to theinsertion loss is first produced and then there is superimposed upon ita resistive pattern also appropriate to that insertion loss and having aresistivity directly related to the required impedance. The resistivepattern is designed to overlap the conductive pattern and the overlapprovided is advantageous in that it eliminates the necessity for veryprecise registration between resistor and conductor elements which isrequired with film attenuators of the prior art.

Well known techniques may be used for depositing the conductive andresistive films, e.g., electroless plating, vacuum evaporation, orscreen printing and firing of cermet materials; however, care must betaken that a resistive film is produced which has a lower resistancevalue than that corresponding to an ideal situation which would not needadjustment, since the adjustment steps described above only aloow anincrease in resistance to be achieved. For this reason, resistive filmswill normally be deposited which are slightly lower in resistivity thanthe required value.

Two embodiments of the invention are now described with reference toFIGS. 3a, 3b, 3c, 3d and FIGS. 4a, 4b, 4c, 4d of the drawings in which:

FIG. 3a represents a plan view of an unbalanced attenuator according tothe invention.

FIG. 3b represents a plan view of the conductive elements of theattenuator of FIG. 3a.

FIG. 3c represents a plan view of the single resistive element of theattenuator of FIG. 3a.

FIG. 3d represents, in schematic form, a circuit equivalent to theattenuator of FIG. 3a.

FIG. 4a represents a plan view of a balanced attenuator according to theinvention.

FIG. 4b represents a plan view of the conductive elements of theattenuator of FIG. 4a.

FIG. 40 represents a plan view of the single resistive element of theattenuator of FIG. 4a.

FIG. 4d represents, in schematic form a circuit equivalent to theattenuator of FIG. 4a.

Referring to FIGS. 3a, 3b, Be, an unbalanced attenuator comprises threemain conductive termination areas, 1, 2, 3 and one further conductorarea 7, deposited on an insulating substrate 11. A single film resistiveelement (FIG. 30) covers the conductor 7 and also the main terminationareas in regions 4, 8, 13, as shown in FIG. 3a.

As shown in the equivalent circuit in FIG. 3d, the attenuator consistsof:

i. A resistive path 6, between terminations l, 2 with a conductor 7constituting its electrical center.

ii. In parallel with the resistive path 6 is a further resistive pathwhich is capable of being adjusted in value by removing part of it bycutting in the direction of the arrow 12.

iii. Between the electrical center conductor 7 and the third maintermination area 3 lies another resistive path 10, also capable of beingadjusted in value by removing part of it in the direction of arrow 9.The input to the attenuator is applied to terminals 1 and 3 and theoutput is taken from terminals 2 and 3.

The attenuator may be manufactured as follows:

Conductive patterns 1, 2, 3, 7 are deposited on an insulating (e. g.,ceramic) substrate 11, by well known techniques such as those previouslymentioned. Conductive patterns 1 and 2 constitute an input and an outputterminal respectively, whereas conductive pattern 3 constitutes acombined input and output terminal. The single resistive film is thendeposited on to the substrate by similar techniques so that it overlapsthe conductor pattern as shown in FIG. 3a. The overlap is such that veryaccurate registration is not required between the conductors and theresistive film. Alternatively, the resistive film may be deposited firstof all on to thesubstrate and the conductive patterns 1, 2, 3, 7 maythen be deposited on top of it. Although not essential, the resistivefilm may be provided with indentations 14 and 15 to indicate theposition where resistance adjustment has subsequently to be carried out.Indentation 14 is midway between terminals 1 and 2. Electricalconnections are made to the terminals 1 and 2 and the resistance valuebetween these terminals is monitored while a portion of the resistivefilm in region 5 is removed in the direction indicated by arrow 12,indentation 14 being a guide to the correct position for removing thefilm. The film is removed by well known techniques, e.g., a jet ofabrasive particles, laser beam, or other suitable grinding or abradingtechniques. The contacts of the resistance monitoring device are thenplaced across input terminal 1 and the combined input and outputterminal 3. A portion of the resistive film in region 10 is removed bysimilar means to those previously employed, the film being cut away inthe direction shown by the arrow 9, starting at the indentation 15.Monitoring in this case may either be by the normal method of resistancemeasurement or by the far more accurate method of actual attenuationmeasurement.

Referring now to FIGS. 4a, 4b, 40, a balanced attenuator comprises fourmain conductive termination areas, constituting two input and two outputterminals 16, 17, 18, 19 and two further conductor areas 20, 21deposited on an insulating substrate 22. A single film resistive element(FIG. 4c) covers the conductors 20, 21 and also overlaps the input andoutput terminals l6, 17, 18, 19 in regions 23, 24, 33, 34 as shown inFIG. 4a. As illustrated in the equivalent circuit in FIG. 4d, theattenuator consists of:

i. A resistive path 26, between input and output terminals 16, 17 withconductor constituting its electrical center.

ii. A resistive path 25 in parallel with path 26 and capable of beingadjusted in value by removing part of it by cutting in the direction ofthe arrow 31.

iii. A resistive path 28, between input and output terminals 18, 19 witha conductor 21 constituting its electrical center.

iv. A resistive path 29 in parallel with path 28 and capable of beingadjusted in value by removing part of it by cutting in the direction ofarrow 32.

v. A further resistive path 27, lying between conductors 20 and 21,which is also capable of being adjusted in value by removing part of itby cutting in the direction of arrow 30.

The input to the attenuator is applied to terminals 16, 18 and theoutput is taken from terminals 17, 19.

The attenuator is manufactured up to the stage immediately prior toadjustment by the techniques described for the unbalanced attenuator.The pattern of conductors may be deposited first of all, followed by thesingle resistive film or the resistive film may be deposited first, withthe conductors on top. Adjustment is then carried out as follows: theresistance value between terminals 16, 17 is monitored and part of thefilm 25 is removed by one of the methods previously mentioned until thedesired value is obtained. The resistive film is cut away in thedirection of arrow 31, an indentation 35 having been formed in theresistive film during the deposition process, together with indentations36, 37 to indicate the midpoint of the resistive film between theconductors on either side. The resistance value between terminals 18, I9is then monitored while part of the film in the region 29 is removed bycutting it away in the direction shown by arrow 32. It is necessary forthe adjusted resistance value to be as close as possible to thatpreviously measured between terminals 16, 17, depending upon theprecision which is required in the attenuator. The resistance valuebetween terminals l6, 18, or between 17, 19, is then monitored whilepart of the film 27 is removed as indicated by the arrow 30. As with theunbalanced attenuator, either resistance measurement or actualattenuation measurement may be made during this part of the adjustmentprocess.

It is well known that the high frequency performance of known flat filmattenuators is superior to that of attenuators comprising discreteresistors, as a result of the reduction of inductance and reactance, butthe performance also tends to vary from pattern to pattern dependingupon the precise layout of the conductors, the positioning of thetermination areas and the amount of adjustment carried out on theresistive elements. In the present invention, it is envisaged that allattenuators of similar insertion loss will be constructed from anidentical pattern designed to cover a wide frequency range with theminimum change of impedance or insertion loss. Thus the high frequencyperformance will be predictable and superior to that of previousdesigns.

The design also has application in continuously variable attenuatornetworks where a single substrate can carry decades of 0.1 dB, 1 dB and10 dB attenuators, directly connected to switches, thus permitting rapidselection of any attenuation or insertion loss between 0.1 dB and 99.9dB in 0.1 dB steps.

Although the above description is solely concerned with attenuators, theprinciple of dividing up a single resistive film element by one or moreshort-circuiting electrically conductive layers combined with removal ofportions of the element may be applied to any networks in which aplurality of film re sistors would normally be used, e.g., in scalingnetworks or ladder networks.

Although the present invention is described herein with particularreference to specific details, it is not intended that such detailsshall be regarded as limitations upon the scope of the invention exceptinsofar as included in the accompanying claims.

I claim:

1. An electrical film attenuator or other film resistor networkcomprising an insulating substrate having adherently mounted thereon asingle film resistive element, said element being provided with at leastthree terminals in electrical contact therewith and, on said element andin electrical contact therewith, an electrically conductive layerdividing up said element into three distinct portions two of which willhave the same electrical resistance value after removal of part of saidelement at a locus midway between two of said terminals, one of whichbeing an input and the other an output terminal, the third portion ofsaid electrical resistive element being shaped so as to enable itselectrical resistance value to be adjusted by removing part of it, eachof said first two portions being in electrical contact with only one ofsaid two terminals, the third portion being electrically connected tothe remaining terminal or terminals.

2. An attenuator or other film resistor network according to claim 1, inwhich a total of three terminals are provided, the third of which beinga combined input and output terminal, and the third portion of the filmresistive element is electrically connected to the third terminal,whereby an unbalanced attenuator results.

3. An electrical film attenuator or other film resistor networkcomprising an insulating substrate, adherently mounted thereon a singlefilm resistive element, said element being provided with four terminalsin electrical contact therewith and, on said element and in electricalcontact therewith, two electrically conductive layers in such a way asto divide up the element into five distinct portions two of which willhave the same electrical resistance value after removal of part of saidelement at a locus midway between a pair of the terminals and twofurther of which will have the same electrical resistance value afterremoval of part of said element at a locus midway between the remainingpair of the terminals, said first pair and second pair of terminals eachbeing an output and an input terminal, the fifth portion extending fromone to the other of said two electrically conductive layers and beingshaped so as to enable its electrical resistance value to be adjusted byremoving part of it, each of said four terminals being as sociated withonly one portion of said two sets of two portions having the sameelectrical resistance value.

4. A method of producing an electrical film attenuator or other filmresistor network, which comprises providing an insulating substrate,adherently mounting thereon a single film resistive element, providingsaid element with at least three terminals in electrical contacttherewith and providing on said element and in electrical contacttherewith an electrically conductive layer in such a way as to divide upthe element into three distinct portions two of which will have the sameelectrical resistance value after removal of part of said element at alocus midway between two of the terminals, one of which being an inputand the other an output terminal, the third portion being shaped so asto enable its electrical resistance value to be adjusted by removingpart of it, each of said first two portions being in electrical contactwith only one of said two terminals, the third portion beingelectrically connected to the remaining terminal or terminals.

5. A method according to claim 4, in which a total of three terminalsare provided, the third of which being a combined input and outputterminal, and the third portion of the resistive film element iselectrically connected to the third terminal.

6. A method according to claim 5, which comprises the further steps of(i) removing part of the resistive film midway between said input andsaid output terminals, or removing a strip of resistive film of uniformwidth from the entire edge of said film between them, and (ii) removingpart of the resistive film to effectively reduce the width and increasethe length of the third portion of the resistive element, whereby it ispossible to measure the attenuation achieved or the electricalresistance value of the network so as to enable compensation for minorerrors produced by step (i).

7. A method of producing an electrical film attenuator or other filmresistor network, which comprises providing an insulating substrate,adherently mounting thereon a single film resistive element, providingsaid element with four terminals in electrical contact therewith andproviding on said element and in electrical contact therewith twoelectrically conductive layers in such a way as to divide up the elementinto five distinct portions two of which will have the same electricalresistance value after removal of part of said element at a locus midwaybetween two of the terminals and two further of which will have the sameelectrical resistance value after removal of part of said element at alocus midway between the remaining of the terminals, said first two andsecond two terminals being one output and one input terminal, the fifthportion extending from one to the other of said two electricallyconductive layers and being shaped so as to enable its electricalresistance value to be adjusted by removing part of it, each of saidfour terminals being associated with only one of said two sets of twoportions having the same electrical resistance value.

8. A method according to claim 7, which comprises the further steps of(i) removing part of the resistive film midway between each of saidfirst mentioned pair of terminals and said second mentioned pair ofterminals or removing a strip of resistive film of uniform width fromthe entire edge of said film between each input and corresponding outputterminals, and (ii) removing part of the resistive film to effectivelyreduce the width and increase the length of the third portion of theresistive element, whereby it is possible to measure the attenuationachieved or the electrical resistance value of the network so as toenable compensation for minor errors produced by step (i).

1. An electrical film attenuator or other film resistor network comprising an insulating substrate having adherently mounted thereon a single film resistive element, said element being provided with at least thrEe terminals in electrical contact therewith and, on said element and in electrical contact therewith, an electrically conductive layer dividing up said element into three distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of said terminals, one of which being an input and the other an output terminal, the third portion of said electrical resistive element being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said first two portions being in electrical contact with only one of said two terminals, the third portion being electrically connected to the remaining terminal or terminals.
 2. An attenuator or other film resistor network according to claim 1, in which a total of three terminals are provided, the third of which being a combined input and output terminal, and the third portion of the film resistive element is electrically connected to the third terminal, whereby an unbalanced attenuator results.
 3. An electrical film attenuator or other film resistor network comprising an insulating substrate, adherently mounted thereon a single film resistive element, said element being provided with four terminals in electrical contact therewith and, on said element and in electrical contact therewith, two electrically conductive layers in such a way as to divide up the element into five distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between a pair of the terminals and two further of which will have the same electrical resistance value after removal of part of said element at a locus midway between the remaining pair of the terminals, said first pair and second pair of terminals each being an output and an input terminal, the fifth portion extending from one to the other of said two electrically conductive layers and being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said four terminals being associated with only one portion of said two sets of two portions having the same electrical resistance value.
 4. A method of producing an electrical film attenuator or other film resistor network, which comprises providing an insulating substrate, adherently mounting thereon a single film resistive element, providing said element with at least three terminals in electrical contact therewith and providing on said element and in electrical contact therewith an electrically conductive layer in such a way as to divide up the element into three distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of the terminals, one of which being an input and the other an output terminal, the third portion being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said first two portions being in electrical contact with only one of said two terminals, the third portion being electrically connected to the remaining terminal or terminals.
 5. A method according to claim 4, in which a total of three terminals are provided, the third of which being a combined input and output terminal, and the third portion of the resistive film element is electrically connected to the third terminal.
 6. A method according to claim 5, which comprises the further steps of (i) removing part of the resistive film midway between said input and said output terminals, or removing a strip of resistive film of uniform width from the entire edge of said film between them, and (ii) removing part of the resistive film to effectively reduce the width and increase the length of the third portion of the resistive element, whereby it is possible to measure the attenuation achieved or the electrical resistance value of the network so as to enable compensation for minor errors produced by step (i).
 7. A method of producing an electrical film attenuator or other film resistor network, which comprises providing an insulating substrate, adherently mounting thereon a single film resistive element, providing said element with four terminals in electrical contact therewith and providing on said element and in electrical contact therewith two electrically conductive layers in such a way as to divide up the element into five distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of the terminals and two further of which will have the same electrical resistance value after removal of part of said element at a locus midway between the remaining of the terminals, said first two and second two terminals being one output and one input terminal, the fifth portion extending from one to the other of said two electrically conductive layers and being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said four terminals being associated with only one of said two sets of two portions having the same electrical resistance value.
 8. A method according to claim 7, which comprises the further steps of (i) removing part of the resistive film midway between each of said first mentioned pair of terminals and said second mentioned pair of terminals, or removing a strip of resistive film of uniform width from the entire edge of said film between each input and corresponding output terminals, and (ii) removing part of the resistive film to effectively reduce the width and increase the length of the third portion of the resistive element, whereby it is possible to measure the attenuation achieved or the electrical resistance value of the network so as to enable compensation for minor errors produced by step (i). 